Where abouts are you...? It would help but there are only a handful of prisons still on analogue... most are tetra and some dmr.... there are some new analogue prison frequencies but they are for works or security.... and by security I dont mean screws

My local one is Airwave, I couldn't speak for yours but I imagine with the nature of their business it's probably in their interests to make listening in as difficult as possible, even for seemingly routine traffic.